Aluminum oxide, tantalum oxide and niobium oxide have conventionally been used as dielectrics of electrolytic capacitors. Although studies have long been conducted on capacitors using titanium dioxide, which have a larger specific dielectric constant than these oxides, for the dielectric (to also be referred to as “titanium capacitors”), these capacitors have not come into practical use due to the problem of large leakage current. A large leakage current is particularly fatal in recently popular metal electrodes effective for reducing impedance in the GHz region since restoration (reoxidation) of electrical leakage cannot be expected when an electrolyte or electrically conductive polymer is used for the cathode in the manner of an electrolytic capacitor.
The following describes previous attempts made to reduce leakage current of titanium capacitors.
It is described in Japanese Examined Patent Application, Second Publication No. S33-5816 (Patent Document 1) that, although a non-aqueous solvent is subjected to anodic oxidation for use as an electrolyte, “a product chemically converted in non-aqueous solvent deteriorates when transferred to an aqueous solution”. Consequently, although not described in the publication, it is clear that a non-aqueous solvent having low electrical conductivity is used as an electrolyte. Perhaps because a non-aqueous solvent is used as an electrolyte, although the leakage current is low, dielectric loss tangent at 10 kHz is 10% or more in all cases.
U.S. Pat. No. 3,126,503 (Patent Document 2) indicates a method for obtaining an anodized film having superior electrical properties by using a titanium alloy containing vanadium, chromium and aluminum. However, the dielectric loss tangent thereof is 1.5% or more.
Japanese Examined Patent Application, Second Publication No. S42-27011 (Patent Document 3) states that a capacitor obtained by anodic oxidation of titanium has leakage current that is greater than that of tantalum or aluminum by two digits or more.
Although Patent Document 3 indicates a method for reducing leakage current by forming a passive layer with nitric acid solution as pretreatment of anodic oxidation, the dielectric loss tangent of the resulting sample is 1.5% or more.
The same inventor as that of Patent Document 3 indicates in Japanese Examined Patent Application, Second Publication No. S42-24103 (Patent Document 4) that the addition of tungsten or molybdenum to titanium reduces leakage current to about one-half that of the case of non-addition. However, even through leakage current is improved by one-half, this is still not adequate for practical use.
Japanese Examined Patent Application, Second Publication No. S43-2649 (Patent Document 5) indicates that leakage current and loss can be reduced by containing barium peroxide or strontium peroxide in a molten salt of sodium nitrite and anodizing at a temperature of 280 to 350° C. However, the dielectric loss tangent at this time is 2.8% or more.
Japanese Examined Patent Application, Second Publication No. S54-1020 (Patent Document 6) indicates that leakage current is reduced by using an alloy containing 20 to 30 atomic percent of aluminum in titanium. However, measurement of electrical properties is carried out in an electrolyte. Typically in electrostatic capacitors, an electrical leakage portion is known to be reanodized and insulated (repair effects) when a direct current voltage is applied in an electrolyte or electrically conductive polymer and the like. Thus, it is presumed that the leakage current decreased due to repair effects in this measurement.
Japanese Unexamined Patent Application, First Publication No. H5-121275 (Patent Document 7) indicates a method for obtaining a capacitor having a satisfactory dielectric loss tangent by adjusting anodic oxidation conditions and carrying out heat treatment thereafter. Since electrical properties are measured using an electrolyte having repair capabilities in this case as well, this is similar to the Patent Document 6 described above.
Moreover, the following lists examples of obtaining a dielectric by forming a compound such as barium titanate on a titanium base material.
Japanese Examined Patent Application, Second Publication No. H6-49950 (Patent Document 8) indicates size reduction of a capacitor by forming Ba1-xSrxTiO3 on titanium. Although Ba1-xSrxTiO3 is shown to be formed by X-ray diffraction, titanium oxide is not confirmed.
Japanese Unexamined Patent Application, First Publication No. H7-86075 (Patent Document 9) indicates a capacitor having a titanium oxide layer and a composite titanium oxide layer on a titanium surface. It is described therein that a titanium oxide thin film is formed at the interface between titanium and barium titanate when a titanium metal substrate, in which a barium titanate thin film is formed on the surface thereof, is fired for 1 hour in air at 700° C. However, an insulating titanium oxide thin film capable of being used as a capacitor cannot be obtained under such firing conditions. This is indicated in, for example, a phase diagram relating to thermal oxidation of titanium in “Phase Diagrams for Electronic Ceramics I: Dielectric Ti, Nb and Ta Oxide Systems” (The American Ceramic Society, pp. 4-8 (Non-Patent Document 1)). Furthermore, insulating titanium oxide refers to titanium dioxide (TiO2). For example, the specific resistance values of various titanium oxides are indicated in Gmelins™ Handbuch Der Anorganischien Chemie Titan”, 1951 Verlag Chemie, GmbH (Non-Patent Document 2). Namely, if titanium remains even after firing at 700° C. in an air atmosphere, TiO will also certainly remain as will Ti2O3. Thus, the reason for the improvement of insulating properties in Patent Document 9 is presumed to be due to the barium titanate thin film having been oxidized resulting in a decrease or crystallization of those portions serving as oxygen vacancies.
Japanese Unexamined Patent Application, First Publication No. H11-172489 (Patent Document 10) indicates a method for forming a barium titanate film by first forming titanium oxide on a substrate followed by anodizing in a barium aqueous solution. In this method, however, the titanium oxide layer was determined to be extinguished by the reaction as shown in FIG. 1.    [Patent Document 1] Japanese Examined Patent Application, Second Publication No. S33-5816    [Patent Document 2] U.S. Pat. No. 3,126,503    [Patent Document 3] Japanese Examined Patent Application, Second Publication No. S42-27011    [Patent Document 4] Japanese Examined Patent Application, Second Publication No. S42-24103    [Patent Document 5] Japanese Examined Patent Application, Second Publication No. S43-2649    [Patent Document 6] Japanese Examined Patent Application, Second Publication No. S54-1020    [Patent Document 7] Japanese Unexamined Patent Application, First Publication No. H5-121275    [Patent Document 8] Japanese Examined Patent Application, Second Publication No. H6-49950    [Patent Document 9] Japanese Unexamined Patent Application, First Publication No. H7-86075    [Patent Document 10] Japanese Unexamined Patent Application, First Publication No. H11-172489    [Non-Patent Document 1] “Phase Diagrams for Electronic Ceramics I: Dielectric Ti, Nb and Ta Oxide Systems” (The American Ceramic Society, pp. 4-8    [Non-Patent Document 2] Gmelins™ Handbuch Der Anorganischien Chemie Titan” (1951) Verlag Chemie, GmbH